a) Field of the Invention
The present invention relates to a nearly telecentric objective lens system for endoscopes, and more specifically to an objective lens system which is composed relatively compactly and so as to correct the negative distortion produced in general telecentric lens systems.
b) Description of the Prior Art
As is well known to those skilled in the art, objective lens systems to be used with fiber scopes, endoscopes and video scopes using single-plate color image pickup devices must be telecentric. Further, it is also known that distortion is aggravated as telecentric objective lens systems for endoscopes have wider angles.
When the entrance and exit pupils are imaged in a condition satisfying the sine condition, and free from spherical aberration, relationship between angle of incidence .theta..sub.1 of the principal ray on the entrance pupil and distortion D(.theta..sub.1)can be expressed by the following formula (i) using magnification .beta..sub.E for the paraxial pupil as a parameter. EQU D(.theta..sub.1)=100.times.[cos.theta..sub.1 /(.sqroot.1-sin.sup.2 .theta..sub.1 /.beta.E.sup.2)-1] (i)
In case of an objective lens system which is strictly telecentric, the magnification for the paraxial pupil has an infinitely large absolute value and the distortion therein is expressed by the following formula (ii), whereby negative distortion is produced: EQU D(.theta..sub.1)=100.times.[cos.theta..sub.1 -1](%) (ii)
For designing an ordinary telecentric objective lens system, however, it is unnecessary to select the magnification .beta..sub.E for the paraxial pupil which strictly has an infinitely large absolute value, but the magnification must have an absolute value .vertline..beta..sub.E .vertline. which is larger than 2. If .vertline..beta..sub.E .vertline. is smaller than 2, the departure from the telecentric condition will be too large to use the objective lens system of interest with endoscopes.
Remarkable negative distortion is produced in objective lens systems which are more or less telecentric. There has been proposed a method to correct this distortion by changing the imaging condition of the pupils with an aspherical surface.
As the conventional examples of objective lens systems for endoscopes which adopt the retrofocus type composition for correcting curvature of field and use aspherical surfaces for correcting distortion, there are known the lens systems disclosed by U.S. Pat. No. 4662725 and U.S. Pat. No. 4867546.
The lens system disclosed by U.S. Pat. No. 4662725 uses, for correcting distortion, an aspherical surface only in the front lens unit which is arranged before a stop. However, the lens system which uses an aspherical surface only in the front lens unit has an outside diameter too large for image height therein as is clear from the embodiments described in the specification of this patent. It is considered that this defect can be attributed to a fact that the correction of the negative distortion with the aspherical surface arranged in the front lens unit produces remarkable adverse influence on the other aberrations, especially curvature of the meridional image surface. When the negative refractive power of the front lens unit is strengthened for preventing the outside diameter of the lens system from being enlarged, the curvature of meridional image surface and the aberrations other than the distortion are aggravated, thereby making the lens system practically usable.
In order to correct the defect of the lens system described above, the lens system disclosed by U.S. Pat. No. 4867546 uses an aspherical surface in each of a front lens unit arranged before a stop and a rear lens unit arranged after the stop for correcting distortion and curvature of field at the same time. Since this lens system is designed so as to correct the negative distortion by using the aspherical surface in the rear lens unit thereof in addition to the aspherical surface arranged in the front lens unit, and correct the influence on curvature of field produced by the use of the aspherical surfaces with the front lens unit and the rear lens unit, the lens system allows the lens units thereof to have powers stronger than those of the lens units arranged in the lens system disclosed by U.S. Pat. No. 4662725 and to be more compact. When the lens system disclosed by U.S. Pat. No. 4867546 is designed as a retrofocus type for correcting Petzval's sum, however, the aspherical surface used in the front lens unit produces an influence.
In a retrofocus type objective lens system for endoscopes, the lens component arranged on the most object side has a negative power, which is inevitably strengthened by designing the lens system more compact. According to the description made in the specification of U.S. Pat. No. 4867546, it is desirable that the aspherical surface used in the front lens unit is located close to the object point for correcting distortion. In other words, it is desirable that the lens component which is arranged on the most object side and has a strong negative power is designed as an aspherical lens component. In all the retrofocus type lens systems out of the embodiments of this conventional example, the negative lens components arranged on the most object side are designed as aspherical lens components. When the negative lens component arranged on the most object side has an aspherical surface, whether the surface is located on the object side or the image side, there is posed a problem described below.
When the surface located on the most object side is designed as an aspherical surface in an objective lens system, it will degrade the water drop-off characteristic required for endoscopes. The water drop-off characteristic means a property to prevent water, viscous liquid, etc. adhering to the lens surfaces in the distal ends of endoscopes from remaining on said surfaces, and is evaluated as a total function including air supply, water feeding and so on for washing the distal ends of endoscopes in the field of digestive organs in which endoscopes are used most widely. When the water drop-off characteristic is degraded, water remaining on the surface of the objective lens system cannot be removed by performing the air supply, water feeding, etc., thereby making it impossible to observe normally through endoscopes. A good water drop-off characteristic is therefore an important requirement for endoscopes.
When the negative lens component located on the most object side has an aspherical surface convex on the object side as in the case of the conventional example described above, the lens component has a high inclination at the outermost marginal portion thereof, and water and/or viscous liquids are apt to remain in the cavity around the boundary between the lens component and the fixing frame therefor. Further, the shape of said aspherical surface is designed so as to have curvature remarkably varied at the marginal portion thereof, and hinders smooth flows of air and water, thereby making it difficult to remove the water, etc. remaining on the marginal portion of the lens component. Accordingly, the objective lens system described as the conventional example can hardly be used with medical endoscopes on which viscous liquids, etc. adhere frequently to the lens surfaces.
When the negative lens component arranged on the most object side has an aspherical surface on the image side, it will be practically impossible to manufacture the lens component. In order to design the lens system of interest compact, the lens component to be arranged on the most object side must have a strongly negative power. A concave surface arranged at the foremost location in an objective lens system is undesirable for correction of distortion, allows foreign matters to be accumulated thereon and hinders the proper function of endoscopes. Accordingly, it is inevitably obliged to impart a strongly negative power to the image side surface of the negative lens component. When this concave surface having the strongly negative power is designed as an aspherical surface which has large departure from the reference sphere thereof to correct distortion, it will be difficult to manufacture this lens component with high precision by press molding. In order to prevent surface precision from being degraded by deformation at the stage of press molding, a body glass material to be manufactured into the aspherical lens component described above is generally shaped so as to have a spherical surface matched to a certain degree with the desired aspherical surface so that the deformation will be as little as possible at the stage of the press molding, eccentricity of stress distribution will be as little as possible between the molding die and the body glass material. However, when the aspherical surface has too large departure from the reference sphere thereof and has a concave shape remarkably varying thickness of the lens component, it will be impossible to distribute stress uniformly at the stage of the press molding. Accordingly, it will be difficult to manufacture the lens component having such an aspherical surface with high precision by the press molding even when the body material thereof is preliminarily shaped.
Though aspherical lens components can be manufactured by methods other than the press molding, these methods cannot provide high productivity, require high manufacturing costs, are applicable only to certain specific types of aspherical surfaces and are not generally usable. The aspherical lens components to be used in the objective lens system for endoscopes according to the present invention are deemed to be manufacturable only by the press molding.